1. Field of the Invention
The present invention relates to an improved arrangement of superconducting elements in a current limiting device in particularly optimized for high voltage applications. Further, the present invention relates to such a current limiting device which can be easily adapted to the particular requirements of a specific application such as specific voltage level of said application etc.
2. Description of Related Art
Superconductors offer a great potential as resistive fault current limiters which enable rapid and effective current limitation, automatic recovery and negligible impedance during normal operation. They are especially an enabling technology for other superconducting applications in the high voltage technology, for example superconducting cables.
Typically a superconducting fault current limiter comprises one or more superconducting elements housed in an insulating housing such as a cryostat, filled with a cooling medium for cooling the superconducting elements below their critical temperature Tc at which they exhibit superconducting properties.
The critical temperature depends on the specific superconducting material. Suitable cooling mediums are for example nitrogen, helium, neon, hydrogen or mixtures thereof in their liquid state.
Preferred superconducting materials are the high temperature superconductors such as BSCCO (bismuth-strontium-calcium-copper-oxide) and YBCO (yttrium-barium-copper-oxide) which have a critical temperature in the range of 67 K and 110 K.
Thus, for these materials liquid nitrogen can be used as cooling medium which is preferred in view of costs.
In the design of a fault current limiter two particular parameters must be taken in consideration.
The first parameter is the nominal current at which the system will be operated as well as the limitation level at which the system shall limit the current.
That is, the superconducting elements must be selected in view of their current carrying capacity as well as in view of their limitation properties.
In order to meet these requirements either one superconducting element or two or more superconducting elements electrically connected in parallel, may be necessary.
The second parameter is the voltage level at which the current limiter shall be operated. By the voltage level the necessary number of superconducting elements which have to be electrically connected in series is determined.
In order to meet these parameters for any specific application a fault current limiter has to be individually designed. Thus, there is a need for a standardized design which allows easy adaption of a fault current limiter to the specific requirements of the respective application.
Further, in a fault current limiter which is to be operated at middle or high voltage level, voltages up to several 100 kV can occur. For avoiding any damage it is necessary that even at such extreme voltage peaks no flash over takes place between the individual structural elements of the current limiter, for example between the superconducting element(s) and the jacket of the cryostat housing, which typically is grounded.
Liquid nitrogen, typically used as cooling medium, also has very good insulation properties and, thus, helps to provide electrical insulation within the fault current limiter.
However, there is a problem, that in fault event the superconducting element(s) are thermally heated up to temperatures exceeding the boiling temperature of the surrounding liquid nitrogen. In the result gaseous nitrogen, that is gas bubbles, are formed within the cryostat. However the electric insulation properties of gaseous nitrogen is significantly less than that of liquid nitrogen. In addition gaseous nitrogen not only has a less break through strength than liquid nitrogen but also leads to reduction, of the electrical field in regions, where the liquid nitrogen is displaced by the gas bubbles, both factors enhancing the risk of flash over. Consequently, for avoiding voltage flash over the distances between the structural elements in the cryostat must be selected greater as would be the case of liquid nitrogen.
Consequently, the required space for the fault current limiter is enhanced.
US 2007/0204632 A1 and JP 04193024 A1 relate to a fault current limiter wherein a first closed cooling bath including the superconducting element is housed within a second closed cooling bath, wherein the pressure exerted onto the first cooling bath is higher than that exerted onto the surrounding second cooling bath for suppressing bubble formation within the first cooling bath on temperature raise.
Also WO 2005/006455 A1 relates to the problem of dissipation of thermal energy from the cooling bath surrounding superconducting elements. To this two separate cooling circuits are provided which are thermally coupled by a heat exchanger. The first cooling circuit serves to cool the superconductor elements. Heat generated is conducted away via the heat exchanger to the second cooling circuit which, in turn, is thermally coupled by a heat exchanger to surroundings for conducting away the heat to the surroundings.
In the first cooling bath liquid Neon (˜27K) and in the second cooling bath liquid Nitrogen (˜77K) is used.
Also EP 1 217 708 A1 relates to a fault current limiter with a first cooling bath for cooling the superconducting elements and a second cooling bath for dissipating heat of the first cooling bath in case of temperature raise.
The second cooling bath serves as a cooling reservoir with a medium having a lower boiling point than the medium of the first cooling bath.
Both bathes and, consequently, the superconductor arrangement are cooled down to a temperature lower than the boiling point of the second cooling bath. In case of temperature raise there is a temperature puffer resulting from the difference in boiling temperature of the bathes.
DE 195 20 205 A1 relates to a known fault current limiter wherein a plurality of plate-shaped limiting elements are connected in series and are immersed into a cryostat 20 filed with cooling medium. Vaporized cooling medium K1 is discharged to a cooling devise 21 for liquefying and provided to the cryostat 20 as liquid cooling medium K2.
However there is still a need for a fault current limiter applying superconducting elements having a space saving arrangement suitable for high voltage application and which can be, nevertheless, safely operated.
Further, there was a demand for a fault current limiter design which can be easily adapted to the specific requirements and condictions of different applications.